Freeze-thaw cycles affect hydrothermal and heavy metal transport mechanisms in porous media: Closed and transient flooded system conditions

Freeze-thaw cycles (FTCs) are one of the main drivers of soil heavy metals (HMs) migration. Soil hydrothermal and HMs migration are closely related, and the hydrothermal environments studied so far are relatively homogeneous and the effects of stagnant water infiltration during freeze-thaw periods are not sufficiently explored. To overcome this limitation, this study sets up FTCs tests under two conditions, a closed system(W2) and a briefly flooded system(W2f). The results showed that the daily mean concentration change rates of HMs in soil layers 4 and 5 in W2f were greater than those in W2 and peaked after the 8th FTCs, and the concentration change rates of Cr and Cd reached 35.52 (mg/kg)/d and 5.02 (mg/kg)/d, respectively (W2f), as well as 25.34 (mg/kg)/d and 3.74 (mg/kg)/d (W2). In contrast, the rate of change in the daily mean concentration of HMs in layer 6 was significantly lower in W2f than in W2. In addition, frequent freezing and thawing altered the preferential flow characteristics of the soil, leading to a slowdown in the rate of HMs migration with increasing number of FTCs. FTCs had little effect on the lateral migration of HMs, but W2f was able to increase lateral migration proximal to the inundation zone. In conclusion, W2f significantly changed the migration pattern of HMs in soil and affected their enrichment in soil, and this study can provide a theoretical basis for the analysis of the migration process of HMs in cold regions.